Primal meat cuts are generally smaller than a side of beef, but larger than the ultimate cut sold to the retail consumer. Primal cuts are prepared at the slaughter house and shipped to retail meat stores or restaurants where they are butchered into smaller cuts of meat commonly referred to as sub-primal meat cuts. Sub-primal cuts, however, may also be prepared at the slaughter house.
Primal and sub-primal cuts are packaged to minimize air (i.e. oxygen) exposure and prevent meat spoilage and discoloration during shipping and handling. One common way to package primal and sub-primal cuts and protect them from the adverse effects of air is to shrink package the cuts in a film having good oxygen barrier properties.
For example, a multilayer film having a barrier layer containing polyvinylidene chloride (PVDC) is a well known packaging material. PVDC is known by those skilled in the art to have excellent oxygen barrier properties. The other layers of the multilayer film function to protect the PVDC layer and provide the requisite low temperature and abrasion resistance properties which the PVDC layer lacks.
There are other types of biaxially oriented heat shrinkable multilayer films which do not require oxygen barrier properties, for example films to be used for shrink wrapping poultry.
Shrinkage properties may be produced in a film by biaxially stretching the film in the machine and transverse direction. The resulting film will heat shrink within a specified range of percentages such as from about 20 to about 50 percent at about 90.degree. C.
Multilayer heat shrinkable film may also be formed into packaging bags. Bags are generally made by transversely sealing a tubular stock of multilayer film and cutting or splitting the tube, or by superimposing flat sheets of multilayer film and sealing on three sides, or by end folding flat sheets and sealing the two sides. A common type of seal used in manufacturing bags is known to those skilled in the art as a hot bar seal in which the adjacent thermoplastic layers are sealed together by application of heat and pressure across the area to be sealed, using contact with opposing bars of which at least one is heated to cause the adjacent layers to fusion bond. For example, bags manufactured from a tube stock generally require one hot bar seal transverse to the tube. This seal is also referred to as a bottom seal. Once the bottom seal is applied, the tube stock is transversely cut to form the mouth of the bag. The strength of seals may be measured by determining the time for a seal to fail when under certain conditions the seal is immersed in hot water at 95.degree. C. i.e., the hot water seal strength ("HWSS") may be measured by a test such as that described as the "restrained shrinkage-seal strength test" in Funderburk et al U.S. Pat. No. 3,900,635.
Once meat or poultry is inserted into the bag, the package is evacuated and the bag mouth must be sealed. At one time, the standard method for sealing a bag was to fasten a clip around the mouth of the bag. More recently, impulse heat sealing techniques have been employed to seal the bag mouth. In general, a bag mouth is impulse sealed by application of heat and pressure using opposing bars at least one of which has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name "impulse") to cause the adjacent film layers to fusion bond. Following the impulse of heat the bars are cooled while continuing to hold the bag inner surfaces together to achieve adequate sealing strength.
One problem with impulse heat sealing is that the film in the seal area often becomes extruded during sealing. This elongation of the product results in thinning of the film and in an extreme situation severing of the thinned film. The latter is known to those skilled in the art as burn-through. One solution to this "burn-through" problem is to irradiate the film prior to manufacture of the bag.
Irradiation of a multilayer film causes the various irradiated layers in the film to cross-link. Under controlled conditions, cross-linking by irradiation not only provides a higher temperature impulse heat sealing range, but also enhances the puncture resistance of the film.
Unfortunately, cross-linked thermoplastic films are not easy to melt, making it difficult for food packagers to achieve strong seals, particularly by impulse sealing the bag mouths after filling with meat or poultry. All of the bag seals (including both the sealed bottom as for example made by the bag manufacturers with a hot bar and the impulse-sealed bag mouth by the food processor) must maintain their integrity when the food-containing package is immersed in hot water to shrink the film against the packaged food. Thus, there is a need for a multilayer film which can be made into a bag having both strong bottom or side seals effected by hot bar sealing or even impulse sealing, and strong mouth seals effected by impulse heat sealing. The film outer layer, which typically contacts the sealing apparatus, should provide high burn-through resistance during sealing and the inner layer should be capable of forming seals having high hot water seal strength.
It is known that heat shrinkable bags for food packaging may be fabricated from multilayer film having a difference in degree of cross-linking of the respective layers of the film. This difference of the degree of cross-linking can be obtained by irradiation of individual layers followed by lamination or extrusion coating. These techniques, however, employ multiple irradiation and/or multiple extrusion or lamination steps.
Canadian Patent 1,125,229 discloses a film structure having a heat sealable inner layer and an outer layer wherein the outer layer is cross-linked to a larger extent than the heat sealable layer. The differential cross-linking is achieved by adding a cross-linking enhancer compound to the outer layer, forming the structure, and then irradiating. The irradiation enhancer allows the irradiation dosage to be lowered to a point where the heat sealable inner layer is not adversely affected in its heat sealing characteristics by the radiation. However, a major problem with this film structure is that by lowering the irradiation dosage, the other beneficial effects which irradiation provides, such as puncture resistance, are surrendered.
Sun U.S. Pat. No. 4,724,176 describes a heat shrinkable bag with an unirradiated inner layer, an oxygen barrier core layer, and an irradiated outer layer. The film used to fabricate this bag is prepared by coextrusion and then controlled irradiation to only cross-link the outer layer. This approach requires precise control of the cross-linking and does not permit enhancement of inner layer puncture strength by cross-linking.
It is also known that manufacturers of thermoplastic resins frequently include limited quantities of antioxidants to improve thermal stability of the resin during storage and processing. These antioxidants tend to retard cross-linking in film extruded from the resin.
The discovery of a differentially cross-linked multilayer film which can be produced by exposure to only one dosage of irradiation and yet still retains the other beneficial effects of irradiation would be widely accepted by those skilled in the art and meet with substantial commercial success.
It is therefore an object of the present invention to provide a differentially cross-linked multilayer film having high hot water seal strength, high burn-through resistance, a broad impulse sealing range and enhanced puncture resistance. It is a further object of the present invention to provide a differentially cross-linked multilayer film having these improvements after exposing the film to only one irradiation dosage. It is still a further object of the present invention to provide a multilayer film structure which can be more easily and efficiently formulated into a packaging bag. It is yet another object of the present invention to provide a meat and poultry packaging bag having both improved bottom and edge seals formed by e.g. hot bar sealing and after food is packaged therein, and improved mouth seals formed by impulse heat sealing. It is still yet another object of the present invention to provide a method for manufacturing a multilayer film having the differential cross-linking characteristics described above.
It is to be understood that these objectives as well as others which are apparent in view of the specification are not to be considered a limitation of the present invention, the scope of which is defined by the appended claims.
In accordance with one aspect of the invention, a tubular article is provided which is formed of biaxially oriented, differentially cross-linked, heat shrinkable multilayer film irradiated to at least about 2 megarad (MR). This film comprises a thermoplastic inner layer and a thermoplastic outer layer, the irradiated inner layer containing an antioxidant in sufficient quantity to adjust its melt flow index to at least about 0.5. The irradiated outer layer has a melt flow index of less than about 0.7, and the ratio of the melt flow indices of the inner layer to the outer layer is at least about 1.5. The inner and outer irradiated layers may be adhered together but in a preferred embodiment the inner and outer layers are separated by an oxygen barrier layer.
At least the irradiated inner layer must be heat sealable to itself, i.e., be capable of fusion bonding by conventional indirect heating means which generate sufficient heat on at least one film contact surface for conduction to the contiguous film contact surface and formation of a bond interface therebetween without loss of the film integrity. Also, the bond interface must be sufficiently thermally stable to prevent gas or liquid leakage therethrough when exposed to above or below ambient temperatures during processing of food within the tube when sealed at both ends, i.e., in bag form. Finally, the bond interface between contiguous inner layers must have sufficient physical strength to withstand the tension resulting from stretching or shrinking due to the food body sealed within the tube.
As used herein, "antioxidant" means an additive to the first layer resin which retards oxidation, i.e., cross-linking, of that layer on irradiation.
Another aspect of the invention relates to a method of manufacturing a tubular article formed of biaxially oriented differentially cross-linked, heat shrinkable multilayer film. In this method a tube of multilayer blown film is formed comprising a thermoplastic inner layer and a thermoplastic outer layer with the inner layer containing an antioxidant. The multilayer film is biaxially oriented to make same heat shrinkable. The biaxially oriented heat shrinkable multilayer film is then irradiated at a dosage of at least about 2 MR to simultaneously adjust the inner layer melt flow index to at least about 0.5 and adjust the outer layer melt flow index to less than about 0.7 such that the ratio of the melt flow indices of the inner layer to the outer layer is at least about 1.5.
A still further aspect of the invention relates to a food storage bag formed of biaxially oriented, differentially cross-linked and heat shrinkable multilayer film irradiated to at least about 2 MR. This film comprises a thermoplastic inner layer and a thermoplastic outer layer, the irradiated inner layer containing an antioxidant in sufficient quantity to adjust its melt flow index to greater than about 0.5. The irradiated outer layer has a melt flow index of less than about 0.7 such that the ratio of the melt flow indices of the inner layer to the outer layer is at least about 1.5. One end of the bag is heat sealed by adhesion between contiguous inner layer surfaces in a direction transverse to the oppositely located side walls of the bag. The mouth end of the bag is impulse heat sealable by fusion bonding between contiguous inner layer surfaces after filling the bag with food.